Add changelog for 3.2.0-RC4

Author: Kordyjan

changelogs/3.2.0-RC4.md

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+# Backported fixes
+
+- Refutable extractor may be an `Apply` tree [#15651](https://github.com/lampepfl/dotty/pull/15651)
+- Bump compiler-interface and capture the `diagnosticCode` [#15565](https://github.com/lampepfl/dotty/pull/15565)
+- Support Mirror synthesis for local and inner classes [#15847](https://github.com/lampepfl/dotty/pull/15847)
+
+# Contributors
+
+Thank you to all the contributors who made this release possible 🎉
+
+According to `git shortlog -sn --no-merges $3.2.0-RC3..3.2.0-RC4` these are:
+
+```
+     2  Chris Kipp
+     2  Paweł Marks
+     1  Jamie Thompson
+     1  odersky
+```